1. Field of the Invention
The invention relates in general to a monitoring system for a semiconductor process. More particularly, the invention relates to a system for monitoring an oxidant concentration in a chemical mechanical polishing (CMP) process.
2. Description of the Related Art
Among the various kinds of planarization processes, chemical mechanical polishing is one of the most straightforward and fastest techniques. Chemical mechanical polishing process is frequently used because it can achieve global planarization. In the chemical mechanical polishing process, a substrate is pressed on a rotating polishing pad. The surface of the substrate is planarized by the polishing pad with the aid of a slurry that contains polishing particles. When the surface to be polished includes a metal layer or an organic polymer layer, an oxidant is added into the slurry, so that the surface is more easily polished and removed.
In the chemical mechanical polishing process, the polishing rate of the substrate is basically proportional to four factors. These four factors include the rotation speed of the polishing pad, the pressure applied to the substrate by the polishing pad, the oxidant concentration of the slurry. The above factors need to be monitored and adjusted according to process requirements to insure that the thickness removed from the polished layer meets the predetermined value.
In the prior art, the oxidant concentration of a slurry is detected using a titration method. That is, a reductive is titrated with the oxidant of the slurry, and another agent is further used to detect the endpoint of titration. This detection method is slow and requires a lot of chemical agents, incurring a long process time, a high cost and labor. Furthermore, as the determination speed of the oxidant concentration is slow, the polishing condition of the polishing table cannot be adjusted in a real time, so that a constant polishing rate cannot be obtained. The thickness removed is thus non-uniform to cause quality difference for products.